Andy, I am cautious as the transformer is private with the closest DB like 15m from the main switchboard. A bump on the 11kV like 6% which is i believe within the DNO fluctuation limits results at a 456V LL which is pretty high. This is a good 14% over 400V.
Just a thought (as usually occurs when you're meant to be thinking about something completely different...) what sort of HV line is your transformer on the end of? If it's a reasonable length, and you have a reasonable baseload, then there will always be some voltage drop along the HV line - so even if the supply end of that live hits the maximum 11kV+6% (or whatever) the transformer primary will always see somewhat less than that.
Of course, if there are times when your LV load is expected to drop to close to zero and the transformer losses are negligible or you're right next door to the 33kV/11kV sub, then I'm probably not helping much again. But it might explain why DNO spec transformers tend to have a slightly higher secondary voltage than the nominal voltages would suggest.
